Electro-hydraulic servovalves are typically used to control fluid flow (rate and direction) from a remote location. A valve of this type will often have multiple stages in which movement of a slide (also known as a spool) of a large valve is controlled through the movement of a much less massive pilot valve. An electrical actuator is usually connected to the pilot valve to control its operation.
There are two common forms of pilot valves used with heavy duty and/or high flow rate electro-hydraulic servovalves. The first type of pilot valve makes use of a movable plate that is located between opposed fluid orifices. Each orifice forms the end of a channel that contains pressurized fluid and that leads to one of two chambers located adjacent opposite sides of the main stage slide. When the plate located between the orifices is shifted by the electrical actuator, it blocks or partially blocks one or the other of the orifices. This causes an obstruction to the fluid exiting the affected orifice and thereby causes a pressure differential to be created in the chambers located adjacent to the ends of the main stage slide. This pressure imbalance causes the slide to shift within its cylinder.
The second type of pilot valve commonly used in heavy duty/high flow rate electro-hydraulic servovalves is very similar in general configuration to the servovalve's main stage valve. The pilot is made up of a slide/spool that is movable within a cylinder. An electrical actuator such as a torque motor is normally used to cause the translation of the pilot stage slide. As the pilot stage slide moves within its cylinder, it uncovers selected ports to thereby enable pressurized fluid to flow past the slide and cause a pressure imbalance to be created in chambers located adjacent opposite ends of the main stage slide. The pressure imbalance causes the main stage slide to shift in the desired direction.
Once the servovalve's main stage slide has been shifted through the action of the pilot valve, it is common for the servovalve to incorporate a feedback mechanism that can return the pilot and main stage valves to a neutral position. In the prior art, the feedback mechanism typically includes a portion that senses the position of either the main stage slide or the load. In addition, the feedback mechanism will employ either a mechanical or fluid connection to cause a repositioning of the pilot valve to thereby cause a rebalancing of the servovalve.
One problem with prior art electro-hydraulic servovalves is experienced when it is necessary or advantageous to adjust the feedback mechanism. In many prior art valves, adjustment of the feedback mechanism is extremely difficult or impossible. In some cases, the feedback mechanism is only accessible after significant disassembly of the valve that may include violating the valve's fluid boundary. If the valve is located in a sealed system, violating the fluid boundary to gain access to the feedback mechanism may necessitate retesting of the entire fluid system. Furthermore, the problematic accessibility of prior art feedback mechanisms significantly exacerbates their maintenance or repair.
A second problem with prior art servovalves that have feedback mechanisms arises due to the mechanism's contact with the system fluid. The fluid can cause corrosion of the mechanism, while entrained particles in the fluid can clog the narrow passages of a fluid-based feedback system or reduce the mobility of components in a mechanical-type feedback system.
A third problem with prior art electro-hydraulic servovalves involves hysteresis effects arising from the structural design of the feedback mechanism. These effects are associated with indirect coupling of the main and pilot stages of the valve and also frictional/dampening forces associated with the functioning of the feedback mechanism.